Various peptides are contained in the blood. Among them, there are peptides having a blood concentration different from the usual blood concentration in the living body in a certain disease state. Such peptides are attracting attention as markers for disease in the field of clinical tests. For example, Simmonds' disease and Sheehan's syndrome caused by the reduced secretion of pituitary hormone can be clinically tested by detecting ACTH as a marker peptide. However, impurities contained in a biological sample to be used for the clinical test may have an influence on the detection of ACTH. Consequently, there is a demand for a method for detecting ACTH which can suppress the influence of impurities in the biological sample.